The demand for very small electrochemical cells has increased with increased use of small, electrically-powered devices, and alkaline and alkaline metal-air electrochemical cells are an increasingly popular choice for powering such devices as hearing appliances and computers. Metal air cells contain an anode and an oxygen cathode, with the oxygen usually obtained from ambient air. The oxygen cathode catalytically promotes the reaction of oxygen with an aqueous neutral or alkaline electrolyte and is not consumed or changed during discharge. As the cathode is extremely compact yet has essentially unlimited capacity, very high energy densities are achieved, resulting from the increased available volume for the anode active material. Consequently, a metal-air cell can provide more watt-hours of electromotive force than a so-called “two-electrode cell” of similar cell size, mass and anode composition that contains both anode- and cathode-active materials inside the cell structure. Metal-air cells have an advantageous watt-hour capacity/mass ratio without regard to size or configuration, which can be, e.g., button cells as in, for example, U.S. Pat. No. 5,721,065 (“the '065 patent”), or cylindrical cells as in, for example, U.S. Pat. No. 6,210,827 (the '827 patent”), each incorporated by reference in its entirety as if set forth herein.
A typical metal air cell such as a zinc air cell contains an anode that includes a zinc alloy powder, a carbon-based air cathode and an alkaline electrolyte with a gelling agent to prevent the electrolyte from creeping through the cell seals. For efficient operation, the anode of a zinc air cell should participate only during current-generating reaction periods. In reality, though, corrosive shelf-discharge side reactions in the alkaline electrolyte can reduce both service and shelf life of alkaline electrochemical systems that use zinc as the anode active material. Much effort has been directed to improving electrochemical reaction efficiency and cell output. Previously, mercury was added to cell anodes to improve corrosion resistance. In recent years, mercury has been replaced by substances that conform to environmental requirements. Small amounts of metals such as lead, indium, and bismuth, and combinations thereof, can effectively improve corrosion behavior of anodic zinc. Certain organic surfactants can provide effective corrosion-inhibiting effect at the metal surface while making the anode sufficiently available for electrochemical oxidation that the cell output can be maintained under heavy cell loading. Surfactants can, like mercury, improve discharge capacity, service life and shelf life, without substantial adverse environmental impact. U.S. Pat. No. 4,857,424 (“the '424 patent”) incorporated by reference herein as if set forth in its entirety, discloses reduced-mercury or mercury-free zinc-manganese dioxide cells containing an organosiliconate type surfactant. Also, the '065 patent discloses a button cell having an anode mix containing zinc metal powder, indium or other compounds, optionally a low level of mercury, a gelling agent and preferably a hydroxyethylcellulose surfactant. The anode disclosed therein sustains longer periods of power production at a relative steady voltage of at least 1.1 volts while protecting the anode metal from corrosion in the alkaline environment.
Similarly, the '827 patent discloses a cylindrical cell having an anode mix containing electrolyte, a gelling agent, particulate zinc, zinc oxide, additives and an organic surfactant comprising hydroxyethylcellulose.
Japanese Patent No. JP10083812 to Toshiba Battery Co. Ltd. discloses providing a high performance zinc alkaline battery containing 0.5-100 ppm of a fluorine-containing surfactant in a non-amalgamated-zinc-alloy-based gelled anode having a viscosity of 100,000 to 300,000 cPs at 25° C.
U.S. Pat. No. 5,382,482 concerns suppressing dendrites and shape change in an alkaline cell by disposing a cross-linked polymer film layer in close proximity to the anode active material, where the polymer film layer can contain, among other polymers, polyoxazoline. The patent does not describe including an oxazoline surfactant in the anode mix of the cell.
Further improvements in performance, such as increased operating voltage, improved discharge profile and reduced sensitivity to open circuit rest in alkaline electrochemical cells, preferably without sacrificing known benefits, are welcome and desired.